Alumina ceramic, either translucent or clear, is particularly suitable as an envelope material for high intensity alkali metal vapor lamps such as high pressure sodium vapor lamps, because it withstands the attack of the vapors of the alkali metals even at high temperatures. In making such lamps, end closures which support the electrodes must be bonded and hermetically sealed to a ceramic tube and a sealing glass or composition is used for this purpose. The closures may take the form of metal end caps which provide direct electrical connection to the electrodes, or of a ceramic plug in which case a metal conductor must be sealed through the plug to provide the electrical connection. The metal having a coefficient of expansion closest to that of alumina ceramic is niobium and it is the one generally used either for the end cap or for the inlead in the case of a ceramic plug.
Desirable properties of a sealing composition are favorable liquidus temperature, wide sealing range, ability to form glasses when melted and cooled rapidly, and stability in the presence of alkali metal vapors at elevated temperatures. The liquidus temperature must of course be above the highest to be encountered during operation of the lamp, but preferably not much above such maximum in order to facilitate manufacture and assure longer furnace life. A composition meeting the foregoing requirements is disclosed in U.S. Pat. No. 3,588,577 McVey et al. It is centered on 45 wt% Al.sub.2 O.sub.3, 36 wt% CaO, 14 wt% BaO, and 5 wt% MgO. This composition, referred to as G45, has been widely used by applicants' assignee in the commercial manufacture of high pressure sodium vapor lamps of the kind using niobium metal end caps. As in most manufacturing operations, some defective products, cracked or leaky seals in this case, are produced when using the G45 sealing composition. Such defective products must be rejected and the proportion thereof is commonly referred to as the shrinkage rate. When the G45 composition is used to seal ceramic end plugs, a somewhat higher shrinkage has been experienced.
The object of the invention is to provide a new composition for sealing metal or ceramic to alumina ceramic and which is superior to the G45 composition. In particular a composition is desired providing a lower shrinkage rate when sealing alumina ceramic parts together in electric lamp manufacture.